1. Field of the Invention
The present invention relates to a layered bandpass filter including three or more resonators that are provided within a layered structure including a plurality of stacked dielectric layers.
2. Description of the Related Art
Today's compact wireless communication apparatuses such as cellular phones increasingly incorporate, in addition to primary wireless communication devices, other communication devices such as those compatible with Bluetooth® standards or those for wireless LANs (local area networks). Thus, there are strong demands for reductions in size and thickness of the communication devices incorporated in the compact wireless communication apparatuses, and accordingly, reductions in size and thickness are also demanded for electronic components for use in such communication devices. The electronic components are also required to be reduced in footprint. To reduce the footprints of the electronic components, it is effective to employ, as the terminal structure of the electronic components, a bottom surface terminal structure such as the land grid array (LGA).
One of the electronic components for communication devices is a bandpass filter for filtering transmission and reception signals. Bandpass filters are also required to be reduced in size, thickness, and footprint. Known as a bandpass filter that is responsive to service frequency bands for various types of communication devices and can be reduced in size and thickness is a layered bandpass filter including a plurality of resonators provided within a layered structure having a plurality of stacked dielectric layers. In the layered bandpass filter, two adjacent resonators are electromagnetically coupled to each other. The electromagnetic coupling includes inductive coupling and capacitive coupling.
Among resonators for use in the layered bandpass filter, well-known is an LC resonator having an inductor formed using a conductor layer such as a strip line. Also known as the LC resonator is the one in which an inductor composed of through holes is connected perpendicular to a conductor layer for a capacitor, as disclosed in U.S. Pat. No. 5,945,892.
The layered bandpass filter has a problem that as the size and thickness of the bandpass filter are reduced, inductive coupling between adjacent resonators may be excessively enhanced to make it difficult to achieve desired characteristics of the bandpass filter.
In the case of the layered bandpass filter in which each resonator is an LC resonator having an inductor formed using a conductor layer, an attempt to reduce the size and thickness of the bandpass filter should result in a reduction in the distance between the conductor layer for the inductor and another conductor layer for a capacitor or the ground This will raise the problem that the magnetic field produced from the conductor layer for the inductor and contributing to inductive coupling between two adjacent resonators is hindered by the other conductor layer, so that the resonators suffer degradation in Q.
The LC resonator having an inductor composed of through holes can provide an enhanced Q because the magnetic field produced from the inductor will be less hindered by other conductor layers. However, when the LC resonator is used for the layered bandpass filter while an attempt is made to reduce the layered bandpass filter in thickness, there is a problem that the through holes constituting the inductor are reduced in length to decrease the inductance of the inductor, thereby causing the resonator to become unable to provide a desired resonant frequency.